Decide how many children to include in the group. Ideally there should between four to six, with a good balance of boys and girls. Being within the same age range is also a good idea. Choose a place to meet, then decide how often and for how long you would like to get together. Also, choose a time of day when the children aren't going to be too tired or hungry.
If you are friendly with any moms or dads from your local playground, ask them if they would be interested in joining. Try posting on local bulletin boards also. Libraries, preschools and churches are great options for finding interested parties.
Figure out a policy concerning sick children. What symptoms should warrant a child staying home? Decide as a group how behavioral issues are to be handled. Discuss any food allergies, and make a plan for snack time. Is it bring your own snack, or are parents going to take turns providing snacks from week to week? The same goes for cleanup. Set a schedule for who is in charge of cleanup from week to week, or simply ask every parent to pitch in each week.